The conversion of esters of carbamic acids to the corresponding isocyanate and alcohol has been extensively investigated. An early Pat. U.S. No. 2,409,712 (1946) shows the pyrolysis of N-substituted carbamic esters in the liquid phase at subatmospheric pressures to give the isocyanate and alcohol. Thus ethoxyethoxyethyl N-lauryl carbamate was pyrolyzed in the liquid phase at 210.degree. C. to 230.degree. C. under high vacuum, i.e. 2 mm. Hg pressure, (no time given) to give lauryl isocyanate and ethoxyethoxyethanol. The yield of isocyanate was 75 percent. The use of such a high vacuum, however, poses problems in commercializing such a process and consequently it is not believed to have been commercialized. Other examples showed much lower yields down to 37 percent.
In an article in the Journal of the American Chemical society, Vol. 80, page 5495 et. seq. (1958), Dyer et al report that monomeric carbamates of the type RNHCOOR' are known to be degraded above 200.degree. C. to the isocyanate and alcohol. The authors show, however, that biscarbamates prepared from methylenebis-(4-phenyl isocyanate) and 1-butanol, 2,2-dimethyl-1-propanol and benzyl alcohol and the polycarbamate from methylene-bis(4-phenyl isocyante) and 1,6-hexane diol when pyrolyzed at 300.degree. C. produced carbon dioxide and the parent alcohol or diol and also yielded amine residues from the benzyl carbamate and polycarbamate.
In another article in the Journal of the American Chemical Society, Vol. 81, page 2138 et. seq. (1959) Dyer et al show that ethyl carbanilate gave phenyl isocyanate (60-75 mole percent based on carbanilate degraded, 44-30 percent degraded, in 6 hours and 4 hours, respectively) and the alcohol when heated at 200.degree. C. under pressure sufficiently low 60-120 mm. Hg to vaporize the alcohol but high enough to retain the isocyanate. Thus even under these conditions other products were formed. At atmospheric pressure no phenylisocyanate was obtained, although 70 percent of the ethyl carbanilate was destroyed. At 250.degree. C. and atmospheric pressure alpha-methylbenzyl carbanilate gave major amounts of aniline, alpha-methylbenzyl aniline, styrene and carbon dioxide.
Dicarbamates such as bis (2-acetoxyethyl) 1,6-hexane dicarbamate according to U.S. Pat. No. 3,054,819 (1962) can be distilled in the presence of a basic catalyst such as tribenzylamine at low pressures, for example, 6 mm. Hg at 215.degree. C. to 260.degree. C. to produce the hexamethylene diisocyanate at a yield of 40 percent. Without the catalyst a yield of only 17 percent was obtained.
A more recent British Pat. No. 1,247,451 (1971) discloses that it was known that organic isocyanates can be produced by the non-catalytic pyrolysis of urethanes at temperatures above 250.degree. C. but the yield is undesirably low, and if lower temperatures are employed the isocyanate and alcohol will react to reform the original urethane. This confirms the findings discussed hereinbefore wherein various expedients such as catalysts and low pressures were employed with specific carbamates. The aforementioned British patent proposes to pyrolyze diethyl toluene-2,4-dicarbamate in the presence of a Lewis acid such as ferric chloride catalyst to produce toluene-2,4-diisocyanate (TDI). A yield of approximately 59 mole percent is shown. In general, the reaction is carried out at from 400.degree. C. to 600.degree. C. in the presence of the catalyst at subatmospheric pressures to form vapors, and condensing the vapors to recover the organic isocyanate product.
Thus although the monocarbamates can be noncatalytically pyrolyzed to the monoisocyanate and alcohol employing high temperatures and low pressures the yields are undesirably low. Likewise certain dicarbamates have been pyrolyzed to the diisocyanate and alcohol but the yields have generally been unsatisfactory for commercial purposes, moreover, in most cases the decomposition products are predominantly amines and carbon dioxide, not the desired diisocyanate. The aforementioned British patent also requires high temperatures and low pressures and in addition a Lewis acid catalyst such as ferric chloride which renders such process highly undesirable from a commercial viewpoint because of the highly corrosive nature of such acids.
The present invention obviates these problems and others encountered by the above illustrated prior processes. It can be carried out at atmospheric, subatmospheric or superatmospheric pressures. The present invention also does not require the use of catalysts and gives extremely high yields of the isocyanate and alcohol. It can be used with esters of carbamic acids in general, but is particularly useful for the production of the mono- and diisocyanates from their corresponding esters of mono- and dicarbamic acids.